Field of the Invention
The present invention relates to a sintering ceramic for stable high-temperature thermistors in the form of a system of matter containing manganese (IV), and to a method for producing such a sintering ceramic.
Semiconducting oxides of the transition elements and combinations thereof are known, for instance, from technical embodiments disclosed in an article in National Technical Report Vol. 34, No. 4, pages 24-34 (1988), entitled Thermistor Sensor for Automotive Uses, based on patent applications such as Published European Application No. 0 149 681 A1 and U.S. Pat. Nos. 4,729,852 and 4,891,158 in the case of the Mn--Ni--Cr--Zn--Zr--Si oxide system, and U.S. Pat. No. 4,324,702 in the case of the Mn--Ni--Cu--Fe--Dr oxide system. Multiphase systems are employed, but without seeking the advantage of forming a uniform phase. The rated resistance R.sub.25 or R.sub.10 O of a thermistor, or in other words the electrical resistance at the temperature T=25.degree. C. and 100.degree. C. and the material constant B of a thermistor that is definitive for the sensitivity of temperature measurement, is adjusted to variable values on the basis of such multiphase systems, in accordance with the following equation: ##EQU1## by carrying out the reaction accordingly in the sintering process, so that at a given offset, production of a certain assortment of thermistors is possible. That kind of procedure generally includes a considerable range of data deviation among the various examples, and especially from one batch to another, since the electrical parameters that characterize the thermistor assume different values depending on the sintered structure attained in the ceramic. In such systems that have been produced, the equilibrium composition of the phases is generally temperature-dependent, which has negative effects on the stability of the electrical parameters over time.
It has been demonstrated that the pure-phase spinel .sub.MgNi II.sub.Mn IV.sub.O.sbsb.4, because of an energetically stable association of the transition metal cations and the lattice places, is characterized by a relatively high B constant of approximately 4600K, and at the same time a rated resistance that is not overly low. The use of a ceramic based on that semiconducting compound as a high-temperature thermistor has been described in German Published, Non-Prosecuted Application DE 42 13 631 A. In that system, upon heating to approximately 700.degree. C., the change in equilibrium composition of phases located next to one another does not occur, so that high stability over time and replicability of the electrical parameters are attained. Above 720.degree., because of the strong polarization of the oxide ions by the Mn.sup.IV cations, decomposition ensues with splitting off of oxygen, and therefore the temperature range within which the semiconducting ceramic based on MgNiMnO.sub.4 can be used is limited.